Brazil has an abundant variety of lignocellulosic fibers due to its abundant biomass and large cultivable territorial extensions [1-7]. It is important to study less common natural fibers to increase their applications [8-15]. Two such fibers, native acai fibers and curaua may be used for polymer based composite materials but also, for other uses. However, vegetable fibers have a hydrophilic character which normally interferes in compatibility with polymer matrices. Composites. This has been overcome by giving surface treatments particularly using chemicals to modify the structure of the fibers whereby enhancing their application capability. While there is very limited published report on the effect of chemical treatments of acai fibers, no report is available of sodium hydroborate (NaBH₄) treatment of both these fibers. This paper presents effect of alkaline treatments with and without sodium borohydride on acai and curaua fibers for possible use in polymer based composites, nanocellulose production and other applications. It was concluded that the alkaline treatment improved thermal stability of both fibers, what is beneficial during the processing stage of composites through extrusion or injection molding.

During the last few decades’ research related to the plant fibers has been driven steadfast due to increasing concern over the environment, depletion of fossil fuels, and climate change [16]. This is exemplified by the fact that increasing wide range of applications, including in engineering sector, of some plant fiber composites have been reported in recent times particularly in North America, Europe, China and Japan [17]. The purpose behind this is to replace the expensive and non-biodegradable synthetic fibers such as glass and aramid fibers in the development of composites with polymer, ceramic and cement based composites [18-24]. This is justified in view of these plant fibers being esthetically interesting and biodegradable besides their exhibiting good thermal insulation and comparable specific strength properties with synthetic fibers. In addition, they are of low cost, durable, sustainable and of renewable sources, which can be used in several segments of the industry and hence they provide an interesting research area and thus considerable efforts are being made to tap their full potential [16,22-30]. Additional attractions for their use in composite industry are their less abrasiveness than that of synthetic fibers and therefore causing low wear and tear to the conventional processing equipment while preparing their polymeric based composites [30,31]. With these unique characteristics, plant fibers offer economical, functional, and environmental advantages.

There have been many studies reported on various fibers of Brazilian origin and their composites, which have been reviewed from time to time [27,32- 33]. Of these, fibers from acai (Euterpe oleracea) palm of genus Euterpe and curaua (Pineapple erectofolius) have been chosen for this study. Figure 1 shows photographs of these plants and their fibers.

Figure 1(a-c): (a)- Euterpe edulis (from Mata Atlântica province, Southern Brazil) palm tree of genus Euterpe; (b)- Acai mesocarp covered by fibers and (c) Red and White curaua plants [34-37].

Acai palm (Figure 1a) is a species of palm tree belonging to Euterpe genre, which is native to Brazil, northern part of South America and Trinidad and mainly cultivated for its fruits and hearts of palm [34]. Acai fibers are obtained from the fruit of the palm trees, which is mostly grown in the Amazonian area and in the Mata Atlântica. It may be noted that there are different Euterpe species grown in these regions. Acai is abundantly produced in the north region of Brazil, especially in the city of Belem, in ParA state, where a large amounts of residues (90%) are generated [38-40]. The acai fibers are present in the cover of the mesocarp as well as in the pulp of the fruit (Figure 1b), which are extracted from these manually.

It is reported by that the thermal behavior presented by the fibers of the mesocarp of the acai is similar to that exhibited by other natural fibers such as coir, sisal, etc., which are already used industrially, particularly in automobile industry [41]. There is no much published work on various properties of these fibers except for a very old report on the morphological properties of the Euterpe edulis seed (from which the fiber is removed) [42].

Although these fibers have been used in handicrafts, papermaking, fertilizer production, oxidant extraction, enzymatic substrate and energy generation, their use in the development of polymer based composites are rarely found in the literature except for one with rubber matrix [39]. However, agro-industrial production of these fibers has a predominant role in the extraction of acai, in view of use of the processed pulp of the fruit for food in several Brazilian states and abroad [34]. Thus, there exist good perspectives for the use of acai fibers in the development of new materials [43-45].

The other fiber taken for this study is the curaua (Ananas erectofolius) fiber from white variety, while there is purple variety also. The fiber used in this study is obtained from the leaves (1.5m long and 4cm wide) of plant (Figure 1c) belonging to the same family of the pineapple and is another plant cultivated in the Amazonian area, in the proximities of Santarem and Belem of ParA [25,46-47].

The curaua fiber is extracted from the sheet using a device called "periquita", or by machine similar to the one used to extract the sisal fiber, with production of about 20-25 kg of fiber per day [22,23,46,48-50]. Morphology and properties of this have been studied by many researchers [22,32,51-53]. According to these studies, this fiber is of light weight and exhibits superior mechanical resistance amongst all the plant-based fibers which is of comparable to those of glass fibers. With these characteristics, the fiber has many applications in industrial sector particularly as reinforcement to develop composites [23,32,52]. Among the benefits of the cultivation of the curaua, it is reported that besides the easy processing of fibers and the possibility to have in consortium with other plantations in reforestation areas, this plant is also provider of an important source of income and jobs in most of needy regions of the country thus providing an opportunity for families and small farmers [25,54-56]. Thus, this fiber is an attractive material both economically and technologically and with attractive characteristics particularly with its adhesion with a polymeric matrix due to the mechanical anchorage, allows its use for the manufacture of composite materials for various industrial applications [47,57-60].

With the above background, studies have been made by the authors to utilize both these fibers in the development of polymer based composites. Of course, composites with plant fibers cannot reach the same strength level as the glass fiber composites in view of their hydrophilic nature, susceptible to degradation, attack by microorganisms, low chemical resistance, etc., leading to incompatibility with the generally hydrophobic polymer matrices, besides their lower thermal resistance and a loss of mechanical properties due to moisture uptake [61,62]. This calls for effecting structural modification of their surface through various methods such as chemical treatments or grafting, and physicochemical treatments [63-65] to achieve superior mechanical performance by the plant fiber based polymer composites. This can be achieved by the optimization of interfacial bonding between the plant fibers and used polymer because the major role is played by the interface through transfer of applied stress and distribution of bonding, which are continued to be understood yet completely [17]. Although there are many publications and reviews dealing with the above aspects, the latest overview is reported recently [17] wherein various factors are presented along with critical suggestions and perspectives. These include: (i) Compatibility between the heterogeneous constituents of these composites, (ii) Different modification approaches to refine the interfacial adhesion with view to overcome the incompatibility between the constituents of these composites, (iii) Mechanisms underlying the interfacial bonding, and finally (iv) the assessment of interface structure and bonding.

Such modifications would result in improvement of interfacial adhesion between the fibers and polymers used leading to increased mechanical properties, bio-durability, and weatherability of the corresponding surface [63,65]. A number of methods of surface modifications (acetylation, mercerization, silanization, coupling agents, etc.,) have been used by various researchers [66-82], which are reviewed from time to time [17,22-24,63,64,83,84]. Of these, one of the most economical, besides modifying the morphology, dimensions and mechanical properties of plant fibers, is reported to be alkaline treatment, also called ‘mercerization’ method [61,66,70]. This also [62,66,85,86] significantly removes the amount of lignin and hemicelluloses that cover the fiber due to the high solubility of hemicelluloses in alkalis. The latter is reported to lead to low rigidity and low dense of interfibrillar regions both resulting in reorganization of cellulose microfibrils at the site of deformation. This reorganization of the microfibrils results in a better distribution of stresses during stretching of the lignocellulosic fibers, giving the fibers better tensile strength. The latter also removes any impurity on the surface of the plant fibers while reducing the microcracks, if any, making the fiber surface to be more uniform and rough, whereby the adhesion between the fiber and the matrix enhanced [85,87].

However, the alkaline treatment is highly aggressive that can decrease the mechanical and thermal properties of the fibers [61,85]. To overcome this, an alternative and less used method has been proposed wherein the sodium borohydrate would act as protecting agent of the fibers [88,89]. This patented process has used (1 % m/v) borohydride ion as a protective agent for sisal fibers treated with different percentages of NaOH for using the fibers in an unsaturated polyester resin [88,89].

 The best results of chemical composition, adhesion, tensile strength, thermal stability and morphology were reported with 5% NaOH. Another study has reported effect of use of sodium borohydride, commonly used as a reducing agent for the reduction of carbonyls, since it is effective under mild and aqueous conditions, which does not occur in the use of other reducing agents such as lithium aluminum hydride [90]. Effect of sodium borohydride on plant fibers through the reduction mechanism of sodium borohydride is well documented [91-93]. It is also reported that the use of borohydride ion as a reducing agent in vegetable fibers has the purpose of reducing or preventing the degradation of the monosaccharide units present in the cellulosic chain by reducing the aldehyde terminal group present in the C-1 of the polysaccharide. These monomeric units provide the fibers with structural stability and are affected by the pH conditions of the alkaline treatment, which significantly reduces the degree of polymerization of the cellulose [88,94].

Based on the above literature survey and background, a study on the effect of use of acai and curaua fibers in the unsaturated polyester resins was taken up as part of Master’s degree dissertation of the first author. The objectives of this study were subjecting both acai and curaua fibers to both alkaline (NaOH) and sodium borohydrate (NaBH₄) treatments and evaluate their effects on various properties and morphologies of these two fibers and their composites. This paper, part of the above work, presents the characterization of both the selected fibers with and without surface modifications (by the above mentioned chemicals) in respect of chemical and thermal properties. Report on studies on the preparation of their composites is being prepared separately.

Preparation of fibers: First, as received acai fibers of 2 cm long were washed thrice and sieved to get highly pure fibers as they were from industrial wastes and had pulp, seeds and peels of fruit residues. On the other hand, as received curaua fibers (white variety), which were of about 80 cm long and in bundles, were separated and cut into about 2 cm long using scissors to be of same size as of acai fibers. Both the fibers were washed and dried in an air circulated hot air oven maintained at 60 ° C for 24h. Then both the fibers were weighed in an analytical balance. Figure 2 shows these fibers before they are used for further studies.

Figure 2 (a-d): Acai fibers: (a)- As received acai mesocarp covered by fibers; (b)- After drying; Curaua fibers: (c)- As received long fibers; (d) Chopped fibers.

Chemical Treatment of The Fibers

The fibers were subjected to two types of chemical treatment; first fibers were given alkali treatment using 5% NaOH and then some of the alkali treated fibers were subjected to 1% NaBH₄ treatment. The alkali treatment was carried out by preparing of a solution containing 5% of NaOH in distilled water using 660 g of NaOH dissolved in 13.2L of water. Then about 100 g of the fibers were immersed in this solution, stirred well and kept for 24 h at room temperature (25 °C). In the case of additional NaBH₄ treatment, 132 g of sodium borohydride was added to the previously prepared 5% NaOH solution, and the ratio of alkaline solution / fibers was maintained at 10g/L following an earlier report [88]. They were then washed with water until neutral pH to remove the remaining solution, if any from the fiber to stop any further reaction. The fibers were then dried in a hot air oven maintained at 60 °C for 24h.

Chemical Characterization

Determination of moisture

Moisture content in the fibers was determined following the TAPPI T550 om-03 standard.

Determination of ashes

Ash contents of both the fibers were determined following TAPPI T211 om-02 standard.

Determination of cellulose, hemicelluloses and lignin

Amount of cellulose, hemicelluloses and lignin present in acai and curaua fibers were determined following the methodology proposed by Silva [95]. 2.2.4. Spectroscopic analysis using Fourier Transform Infrared (FTIR)

Spectroscopic analysis using Fourier Transform Infrared (FTIR)

The spectroscopic analysis using total reflectance method was carried out in a Parkin Elmer FTIR spectrometer (Model: Espectrum One B) using attenuated total reflectance (ATR) with crystal attachment. The fibers were pressed on top of a crystal and spectra were obtained with 32 scans for each fiber sample using a resolution of 4 cm^-1 in the wave number range of 4000 to 550 cm^-1. Three repetitions for each sample were made to check for the reproducibility of the obtained results.

Thermogravimetric Analysis (TGA)

The thermal characterization of both the fibers with and without the alkaline treatments was carried out in TGA testing equipment (Make: TA Instruments No. / 10198/TA; Model: Q50) in the temperature range varying between 25 and 650 ºC, in steps of 10 °C/min in an inert nitrogen atmosphere.

Chemical Characterization of The Fibers

Chemical characterization of any lignocellulosic fibers is essential and relevant for finding their uses. In the case of lignocellulosic fibers, it is essential for using them as reinforcements in various natural and synthetic polymeric matrices in view of understanding the interface between the matrix and the filler/reinforcement. This interface indicates chemical compatibility between the polymer matrix and the fiber as an import role in dictating good adhesion and consequently good mechanical properties is dictated by the interface.

Results of chemical characterization of the açai and curauá fibers with and without alkaline treatment (5% NaOH) and (5% NaOH + 1% NaBH₄) with the addition of a protecting agent are given in Table 1(a & b) respectively. At the outset, it can be seen from the tables that total constituents of the fibers with or without any chemical treatments would not add up to 100%. In the case of fibers without any treatment, the above can be attributed to the fact that several factors (nature, collection place, age, genetic variety, species, type of soil, growth conditions, etc.) affect the compositionof lignocellulosic fibers [96-102]. In the case of chemically treated fibers, one can expect lower values of constituents, because these are affected by the chemicals used [81].

Table 1(a): Chemical Composition of the acai fibers before and after chemical treatments.

• Cellulose content of the lignocellulosic fibers increased with the alkaline treatment.
• Chemical treatments with 5 % NaOH and 5 % NaOH + 1% NaBH₄ were effective in the reduction of amorphous phase (hemicelluloses) while increasing the crystalline phase (Cellulose) of both acai and curaua fibers.
• The above results were corroborated by the FTIR studies of these fibers wherein spectral bands related to hemicelluloses disappeared on chemical treatments of these fibers.
• Thermal studies revealed enhancement of degradation temperatures of both the fibers on chemical treatments used in this study thus underlining the improved thermal stability in the fibers.
• Obtained results clearly suggest the reinforcement potential of both the fibers is greater in relation to the untreated fibers whereby possibility of obtaining better strength and thermal properties of polymer composites can be expected when these two fibers with chemical treatments are used as reinforcements.

Funding: The first author (Larissa R. Gehlen) received fellowship from CNPq during her Master degree during which time this study was undertaken with Process N° 133777/2012-2. Similarly, Prof. Thais has received funding from National Council for Scientific and Technological Research (CNPq, Process No.302459/2023-5.

Acknowledgments

The authors would like to thank Ruth Santana, Elisane Koller, Suelen Souza and Claudio Nunes Jr for their help in the laboratory or discussion . They also thank the Univille (Universidade da Regiao de Joinville, Santa Catarina, Rua Paulo Malschitzk, 10 - Zona Industrial Norte, Joinville - SC, 89219-710, Brazil) for permitting to use the facilities during the course of this study, Center of Support to Projects of Community Action in Belem - PA (CEAPAC) and Alicon Agroindustrial Company Garuva-SC for the supply of curaua and acai fibers used in the present study. One of the authors (Dr. KGS) would like to express sincere thanks to Poornaprajna Institute for Scientific Research (PPISR), Bengaluru, with whom he has been associated, for their encouragement.

Conflicts of Interest

On behalf of all the authors, it is declared that there are no financial and personal relationships with other people or organizations through the following: employment, consultancies, stock ownership, honoraria, paid expert testimony, patent applications/registrations, and grants or other funding, which would inappropriately influence this study.

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